Documentations

C++

Bruce Eckel's Free Online Books
Thinking in C++, 2nd ed. Volume 1
Thinking in C++, 2nd ed. Volume 2

Java

Bruce Eckel's Free Online Books
Thinking in Java, 1st edition
Thinking in Java, 2nd edition
Thinking in Java, 3nd edition

HTML

HTML Documentation
The World Wide Web Consortium

MySQL

MySQL Documentation
Developer Zone

GIS Software

Basic ArcMap and ArcGIS Documentation
Harvard University

PCRaster

PCRaster Documentation
Utrecht University